The recovery of precious metals, primarily gold and silver, from low grade tailings or dump ore has been known for many years. Typically, the low grade ore is crushed to a uniform size then agglomerated prior to construction into heaps. The heaps are constructed upon impervious leach pads. A leach solution such as sodium cyanide is sprayed onto the heaps and allowed to percolate through the heaps. The percolating leachate dissolves metals such as gold, silver, copper, etc., in the heap. The pregnant leach solution is collected and the gold and silver recovered via a zinc precipitation Merrill-Crowe process or via a carbon adsorption/desorption process. In the activated carbon desorption process the gold is stripped from the carbon by increasing the temperature, cyanide concentration and pH of the pregnant solution. The pregnant stripper solution is recirculated through either an electrowinning circuit to recover gold on a steel wool electrode or through a Merrill-Crowe circuit to recover gold precipitate as a filter cake.
In the use of zinc precipitation of metals from such cyanide solutions, it is essential that the solution be clarified to approximately 5 ppm solids or less, and deaerated to about 1 ppm oxygen. There must be adequate zinc, an appropriate portion of lead nitrate, sufficient free cyanide, proper solution pH's and an appropriate filter media such as diatomaceous earth.
The cyanide solution will dissolve other metals in addition to the gold and silver sought in such mining operations Copper and calcium as well as others may be dissolved by the leach solution and result in a decreased overall yield of gold and silver. Calcium and other scale forming metals can result in scale formation on the stripping circuit heat transfer surfaces or in the filter presses of a Merrill-Crowe recovery process.
The use of chelation chemistry to inhibit scale deposition is known. To alleviate scale deposition many commercial antiscalants are available. These include maleic polymers, acrylic polymers, phosphonates and combinations thereof. Anti-scalant agents comprising chelating agents and organic dispersants have also been employed. Such antiscalants sequester calcium and render it non-reactive thus reducing scale deposition. The organic dispersants keep previously formed scale crystals free-flowing. The chemistry of chelation is well known as are common chelating agents such as ethylenediaminetetra acetic acid, nitrilotriacetic acid and hydroxyethylethylenediaminetriacetic acid. The reaction of chelation is a stoichiometric reaction. Chelating agents will complex certain metallic ions preferentially to others. PG,4
The chelation constant K.sub.ma predicts the strength of the metal - ligand complex which forms. The most strongly complexed ion is chelated before the next most strongly complexed ion is chelated. Thus, when a number of species are present which will be chelated by the chosen scale control additive, the stoichiometric amount of scale control agent required may be excessive.
Many copper oxide and sulfide minerals are quite soluble in cyanide solutions. Copper forms several cyano complexes and the cyanide in these complexes is unavailable for gold dissolution. However, much of it reports as a free cyanide when the leach solution is analyzed by the standard silver nitrate titration procedure. This means that a solution which apparently contains a sufficiency of free cyanide for gold recovery may in fact give incomplete gold dissolution.
Copper also causes a decrease in the yield of gold and silver because it precipitates, along with gold and silver, when powdered zinc is added to the pregnant solution. Thus copper will dilute the dore bullion. Copper will also increase cyanide consumption.
Because, of these undesirable effects caused by the presence of copper, ore containing more than about 0.3% copper is often treated as gold containing copper ore. The copper is floated into a concentrate and the neutralized flotation tails cyanided to recover the gold.
In gold ores where the amount of copper is not sufficient to justify the recovery of copper, the copper is not recovered separate from gold and silver and decreases the overall yield of gold and silver in the dore bullion.